<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
<input type="button" value="按钮" id="btn">
<img src="../images/4.jpg" alt="" id="im">
<script>
    function my$(id) {
        return document.getElementById(id);
    }

    my$("btn").onclick = function () {
        my$("im").src = "../images/5.jpg";
    };


</script>
</body>
</html>